01b This group can be detected by eye and is an excellent candidate for a coeval stellar population. The CMD and isochrones indicate the presence of main-sequence stars only with an age of 15Myr or younger. Prominent nebular emission suggests an upper age limit of 10Myr. It is located around the SGS.
01d This is a dispersed group around the SGS. The CMD is consistent with a population of <=40Myr. It may well be a remnant of the star-forming episode that created the SGS. The group size is difficult to estimate since the density profile does not flatten.
01z This group is located within the SGS and is easily found by HOP. It contrasts well with the surrounding diffuse field on the density profile. The CMD indicates a very young cluster of 10-15Myr. This is consistent with the residual nebular emission located in a shell around the group. The group is also characterized with a denser core superimposed onto a more dispersed population of stars, as described by Maiz-Apellaniz (2001ApJ...563..151M). The core is contaminated by a foreground star.
02b This is a good case of a very dispersed group that does not stand out well from the diffuse field on the CMD, but still shows a clear overdensity on the density profile. Its coeval property is disputable. The group is in the peripheral region of SGS and cannot be much younger than 40Myr.
02z This is a large and fairly dispersed young group with faint nebular emission. The CMD suggests a dual population, one with several evolved red stars of ~30-40Myr plus another younger one of ~10Myr. The size of this dispersed group is not clear, since the background level is quite high in the SGS region.
03a This small group has a fairly compact core, although it contains few stars. They are very consistent with a population of ~40Myr, with two evolved red stars on the isochrone.
03z This is a fairly dispersed group within the SGS which shows faint nebular emission that may or may not be associated with it. It does not stand out particularly well from its surroundings according to the density profile. The CMD suggests a dual population, one around 40Myr and a younger one on the 10-16Myr isochrone.
04z This young group is at the center of the SGS and is possibly responsible for the HI shell. No nebular emission is associated with this group. The CMD shows a few objects on evolutionary tracks younger than 40Myr, but they are likely stellar blends or multiple unresolved stars since there is a highly dense core at the center of the group.
05a This group contains a small number of stars, but one is very bright and two are surrounded by faint nebular emission, which helps to constrain the age to no more than 10-15Myr. The density profile is fairly sharp and indicates a group size of ~40pc.
05z This is a very interesting group within the SGS with a very bright and compact core. The density profile shows a hole at the center since no individual star can be resolved. The CMD indicates an age around 15-20Myr, if we ignore the brighter sources that are most likely stellar blends. It is interesting to note the high background level in this region of the galaxy.
06z This is probably a random grouping within the local diffuse field of the SGS. There is nothing obvious on the density profile. If this is typical of the local diffuse field, then it indicates a stellar population of 30-40Myr.
07c This group surrounds a young and bright HII region triggered by the SGS. The CMD shows a stellar group of about 15Myr. However, it may be younger because of the nebular emission seen with this group, although the gas may also be ionized by the nearby HII region. The density profile shows an irregular profile, with an overdensity at the center and a lack of stars at a radius of ~30pc compared to the general background density. It may be a combined effect of the general irregular shape of the loose stellar group and the high diffuse field level.
08a This group is very sparse and barely stands out on the density profile. It may well be part of the local diffuse field related to the SGS generation or simply an asterism. It is at most 20-40Myr.
11a This is another sparse group, as shown by the density profile. One bright blue star on the CMD suggests an age of ~30Myr or younger. Nebular emission is seen around one star only.
11c This group, next to 11a, contains a very small number of stars and shows a faint and extended nebular emission. This group contains a few blue stars clustered together seen with HOP, but this does not show up in the density profile. The CMD suggests an age of at most 40Myr, but the faint nebular emission associated with it indicates an age younger than 10Myr.
13a This is a very young group with nebular emission easily picked up by HOP. This group stands out easily from the diffuse field. Its density profile would be consistent with an OB association or a scaled OB association (Maiz-Apellaniz et al., 2004AJ....128.1196M), depending on its mass. Its shape and size are difficult to establish since the density profile does not flatten.
13z This is probably the brightest group associated with the SGS. It is very large (90pc) and elongated. It displays strong nebular emission clearly related to a <10Myr population, which is also consistent with the CMD. The core is very crowded and contains at least five highly compact sub-groups. The density profile confirms the size and shape of the imposing group.
15a This group is very sparse. It is located near the detector edge and may be part of a bigger system. A few red stars on the CMD suggests an age around 40Myr. A bright yellow star appears on the instability strip of the CMD and may well be a Cepheid star ({alpha}=10:29:00.77, {delta}=+68:28:29.0 in J2000). The density profile shows a hole in the center of the group, which may indicate that the group formed along filaments within the molecular cloud.
15z This is a very young and bright group for which most stars cannot be resolved. It has a strong nebular emission, and is therefore younger than 10Myr. Four yellow stars on the CMD are good Cepheid candidates (in J2000: {alpha}1=10:28:47.9, {delta}1=+68:28:04.9; {alpha}2=10:28:48.4, {delta}2=+68:28:05.0; {alpha}3=10:28:47.9, {delta}3=+68:28:05.4; {alpha}4=10:28:48.5, {delta}4=+68:28:01.8).
16c This is an isolated group with a CMD generally consistent with an age of 40Myr. One star may be a Cepheid on the instability strip ({alpha}=10:28:55.5, {delta}=+68:27:57.9 in J2000). One very bright blue star is not consistent with a 40Myr age, and may be a stellar blend.
18c This group is close to an HII region and contains several smaller subgroups. This picture would fit well within the hierarchical star formation theory (i.e., Efremov, 1995AJ....110.2757E; Efremov & Elmegreen, 1998MNRAS.299..588E; Elmegreen et al., 2000prpl.conf..179E; Elmegreen, 2002ApJ...564..773E). The group is dispersed but quite rich in its number of bright blue stars. The CMD is consistent with an age of 20-40Myr, although each subgroup could have slightly different ages, as suggested by a small group of stars with clear nebular emission. Several background galaxies are seen in this region of IC 2574. A bright yellow star located on the instability strip of the CMD is a good Cepheid candidate ({alpha}=10:28:53.7, {delta}=+68:28:35.5 in J2000).
18z This is a small group in the SGS of a few bright stars. Some sources may be the result of blends. According to the CMD, this group is at most 20Myr.
19z This is a fairly small and dispersed group in the SGS without nebular emission. The CMD indicates a population younger than 16Myr. The density profile is quite noisy.
20z This is a dispersed group in the SGS without nebular emission. The CMD indicates an age around 10-15Myr, together with a few evolved red stars on the ~40Myr isochrone. This group does not stand out much from the high diffuse field level. It may be an asterism.
21c This is an extremely dispersed group which is basically part of the diffuse field population, possibly an asterism, with an age of 100Myr.
21z This is a bright star-forming group within the SGS with strong nebular emission. The nebular emission is consistent with a CMD age of 10Myr, at most. This group contains several sub-groups, some of which display a very high stellar density with unresolved stars.
22z This is a bright and young stellar group in the SGS and probably linked to group 21z. The strong nebular emission and the CMD suggest an age younger than 10Myr. A bright red star is observed in this group and is probably part of the underlying diffuse field population. The noisy density profile suggests a size between 15 and 30pc.
23c This is a fairly isolated, young, and compact group, with a low stellar background level. It displays strong nebular emission around a few stars, which indicate an age under 10Myr.
23z This relatively small group (20-30pc) stands out relatively well from the density profile. It produces a faint and diffuse nebular emission, in agreement with the CMD age younger than 15Myr.
24z This group shows a very bright and compact component asymmetrically overlaid on a more diffuse distribution of stars. Due to the asymmetry, it is difficult to precisely establish the size of the group because it interferes with the density profile. As a result of the strong nebular emission, the CMD displays multiple stars bluer than the main sequence. None of the brightest stars are red and evolved, consistent with the young age of <10Myr already suggested by the nebular emission.
25c This is a fairly compact and young group with bright nebular emission, setting its age under 10Myr which is consistent with the CMD. This is part of a larger group that includes several substructures. A visual inspection indicates signs that the central part may contain a dense core with unresolved stars. The CMD shows two yellow stars that are good candidates for being Cepheid variables ({alpha}1=10:28:37.40, {delta}1=+68:27:58.3; {alpha}2=10:28:37.45, {delta}2=+68:27:57.6).
25e This young and small group with nebular emission is part of the larger group with 25c. It shows elongated features that may be unresolved stars. The density profile suggests a size between 10 and 40pc.
25g This group contains very few stars, as can be seen in the CMD and by the noise on the density profile. It is part of the larger group together with 25c and 25e. It displays some nebular emission, in agreement with the CMD age estimation.
25h This is at the center of the larger group that includes 25c, 25e, and 25g. Some nebular emission is detected in the H{alpha} image at the location of this group, but it is not noticeable in the broadband images. This provides an age not much older than 10-15Myr for this group, consistent with the CMD.
25i This small HOP group is part of the larger group with 25cegh, but is much more sparse and does not show signs of nebular emission. It barely stands out of the diffuse field population, as can seen on the image and the density profile. The CMD strongly suggests an age around 40Myr.
28b This is a very sparse group with faint nebular emission, located at the edge of the SGS. The presence of nebular emission is consistent with the few bright blue stars on the main sequence of the <16Myr Padova isochrones.
29f This is a very small group around an HII region. This is actually an OB group next to a more intense star-forming system. There is strong nebular emission associated with one or a few stars.
30b This group is fairly isolated and sparse, and yet still pretty young with two bright stars, one blue and one red, that allow us to constrain the age to 10-40Myr at most. A very faint nebular emission can be see on the F555W image. The center of the group and its size are very difficult to estimate since the system is very asymmetric.
30d This is an interesting group because it is fairly isolated, not particularly young (~100Myr), and yet it stands out from the diffuse field population in the CMD and in the density profile. It is an excellent candidate for a dissolving system.
34b This is a group with a very few number of bright blue stars, but it stands out from the diffuse field population on the density profile. Its age is difficult to estimate, with only one star brighter than F555W=24mag on the 40Myr isochrone. This bright central star shows clear emission in the H{alpha} and may also be a stellar blend.
38c This is a nice sparse group with a low number of stars for which its age is between 10 and 40Myr. However, the brightest star is possibly a contaminating foreground star, which would bring the age to about 40Myr.
40b This group was easily picked up by HOP, but curiously the density profile does not show in it anything more compact than the surrounding diffuse field. It is a very young group with no nebular emission, and not compact at all. The group is clearly distinct from the older diffuse field population on the CMD, and its age is <=15Myr.
41a This group stands out with HOP because of the concentration of bright blue stars, but it is as sparse as the local diffuse field density according to the density profile. It is quite young, probably at most 15Myr, but a few yellow stars falling on the 40Myr isochrone give cause for doubt. One of those yellow stars is associated with compact nebular emission that is better seen in the H{alpha} image.
47b This is a very sparse group identified by HOP, with a density profile suggesting an overdensity. The CMD shows a few bright blue stars consistent with an age of 20Myr or younger. It may well be an asterism.
49a This group is probably part of the diffuse field population. The density profile is quite flat but it also has a quite high diffuse field density. The CMD shows many stars on the 100Myr isochrone that are fairly well separated from the field stars. It is a good candidate for a highly dissolved system. Its size and shape are not possible to establish.
49c This is a very small group with faint nebular emission. It contains a few stars that are consistent with a ~20Myr isochrone or younger.
50z This group belongs to the giant HII region and is associated with strong nebular emission. This emission is in agreement with the CMD age of <10Myr.
51a This is an isolated and dispersed group that includes one bright blue star, consistent with a 15Myr isochrone or younger. Without that one star, an age closer to 100Myr would be appropriate given the CMD. The group barely stands out from the diffuse stellar field population. It may well be an asterism.
51z This group within the giant HII region shows some nebular emission in its outskirts. This is consistent with an age of 10-15Myr suggested by the CMD fitting.
52z This small group (~20pc) displays strong nebular emission and contributes to the brightness of the giant HII region. This age is consistent with the CMD fitting of <40Myr and is based on a fairly limited amount of stars.
53z This group is fairly compact and does not show obvious nebular emission directly associated with it. The CMD and isochrone fitting indicate an age of 10-15Myr, although the few bright blue stars may be stellar blends. It is easily picked up with HOP but is not clear on the density profile.
54c This is a very isolated and small group that stands out very well from the diffuse field population on the density profile. It was easy to identify it with HOP. According to the CMD, it is probably about 40-60Myr. Two of the brightest stars are located on the instability strip and may be Cepheids, depending on their reddening (in J2000: {alpha}1=10:29:00.58, {delta}1=+68:26:43.13; {alpha}2=10:29:00.81, {delta}2=+68:26:42.9).
54z This is a dispersed group within the giant HII region. Faint, diffuse nebular emission can be detected, which is consistent with a CMD fitting of <15Myr. The ~40pc system is well seen on the density profile.
55b This is an isolated and small dispersed group. The density profile shows a clear detection. However, the age is somewhat difficult to estimate based on the CMD due to the very low number of bright stars in the group and the fact that there is no bright yellow or red star to constrain the isochrones. The brightest blue star, if it is not a multiple system, suggests an age of 20Myr or younger. If we ignore the brightest blue star, then we obtain an age of <=50Myr.
56a This group is located at the very edge of the detector, and may not be complete. The CMD suggests and age younger than 15Myr, consistent with the nebular emission observed in the F555W filter and the H{alpha} image.
57c This is a sparse group clearly detected by HOP, but it does not stand out on either the density profile or the CMD. The CMD shows a few stars on the main sequence that are consistent with a <=30Myr isochrone, but two very red stars may extend the age up to ~70Myr, depending on the reddening. This group is a good candidate for a highly dissolved coeval system.
58a This group is very small. It is located right next to a compact star cluster with unresolved stars. The group does not show signs of nebular emission and two stars on the CMD suggest an age estimation of <=20Myr. The group does not stand out much on the density profile.
60e This small group is fairly isolated and dispersed, but does not have bright blue stars standing out from the older diffuse field in the CMD. The density profile shows a small overdensity from the diffuse field. The age of its brightest stars would be between 40 and 100Myr, according to the isochrones.
61c This is a very small group with a few stars that could fit a <=40Myr system. The density profile is very noisy.
62c This group is very sparse and would not have been seen without HOP. It is located near the detector edge. The CMD suggests a population of ~40Myr. The density profile is unreliable at r>120pc because it falls outside of the detector. This is a good candidate for a dissolving coeval system.
67d This is a sparse group with few stars, and is close to the group 40b. The CMD suggests an age of 40Myr, although it is contaminated by stars of older generations. The density profile shows an overdensity at this location within a 40pc radius.
69e This group clearly stands out from the diffuse field population, even though the density profile is biased by the edge of the detector for r>60pc. The CMD fits an age of 20Myr or younger. No nebular emission is detected. The group is contaminated by background galaxies. One bright yellow star may be a Cepheid ({alpha}=10:28:38.62, {delta}=+68:25:41.1 in J2000).
70b This is an extremely dispersed group with significant contamination. It is a good candidate for a dissolving cluster. It stands out marginally from the density profile. Its size and shape are very difficult to establish. The star at {alpha}=10:28:41.28 {delta}=+68:25:52.4 may be a Cepheid.
71a This is a nice loose group. It is overdense on the density profile and there are several yellow and red stars on the 40Myr isochrone. One blue star shows very faint nebular emission. Another is a good Cepheid candidate given its CMD location on the instability strip ({alpha}=10:28:44.73, {delta}=+68:25:44.17 in J2000).
71b This group, located next to 71a, is very sparse. It is about 30Myr, according to the CMD and isochrones. The stellar overdensity is relatively weak.
71c This dispersed group is located near 71a and 71b. There are many bright blue stars on the CMD but they do not clearly converge toward a consistent age. It could be around 50Myr if we ignore the brightest star or <=15Myr if we assume that a few yellow stars are in fact main-sequence stars affected by some level of reddening. There is a faint detection of nebular emission in the F555W filter as well as in the H{alpha} image, which strengthens the younger age hypothesis. The density profile suggests a weak overdensity in this location.
71e This group is a good candidate for a dissolving system. It is contaminated by a background galaxy. Several stars lying on the ~40-60Myr isochrone separate fairly well from the diffuse field population. Two stars may be on the instability strip, depending on the reddening ({alpha}1=10:28:43.94, {delta}1=+68:25:52.5; {alpha}2=10:28:44.67, {delta}2=+68:25:51.8 in J2000)
72b This is a nice young group, rich in bright blue stars with nebular emission seen in H{alpha}, with an age or 10Myr at most, which is supported by the CMD. The density profile shows a clear overdensity that stands out from the diffuse field population.
74c This is a nice group, fairly compact, and showing clear nebular emission, which makes it younger than 10Myr. The density profile shows a clear stellar overdensity. The group may be linked to the giant HII region.
75e This is a nice sparse group that stands out clearly on the stellar density profile. It is associated with some nebular emission inferring an age younger than 10Myr, as supported by the CMD. It is probably linked to the giant HII region.
76d This group stands out from the density profile as well as on the CMD. It is about 40Myr, but may be mixed with a ~100Myr population, depending on the reddening of two yellow stars. It is located next to a compact star cluster and is probably linked to the giant HII region.
77a This group is fairly dispersed and shows several stars consistent with a ~40Myr isochrone. However, the group is located in the outskirts of the giant HII region and the density profile is not clear about an overdensity. The group is probably part of the formation process related to the HII region.
80c This group is sparse and located in the outskirts of the large HII region. The group stands out both in the CMD and the density profile. A few evolved bright stars are consistent with the 30-40Myr isochrone. No nebular emission is seen with the group.
81c This group is located at the very edge of the detector. The density profile is not fully reliable because of that. The CMD however shows a stellar population of 40Myr or younger. No nebular emission is seen.
83d This group is made up of a good number of bright blue stars and looks pretty compact on the F435W image. Interestingly, the density profile does not show a clear overdensity. The group is associated with diffuse nebular emission and may be part of a bigger group. The young stars stand out very nicely on the CMD, and the age is estimated to <10Myr.